The present invention relates to a wideband, high-sensitivity vector ammeter. Vector ammeters according to the invention can be used as wideband, high-sensitivity voltmeters and impedance meters The vector ammeter referred to herein is a measuring instrument which produces electric current in a certain ratio to other current or voltage of the same frequency.
Wideband, high-sensitivity vector ammeters find wide application. When a resistor is connected to the input terminal of such an ammeter to convert voltage into electric current, it is possible to know the resistance value of the resistor and to measure the input voltage.
Also, wideband, high-sensitivity vector ammeters are used as main portions of wideband half bridge circuits of electronic element measuring instruments, such as the HP 4274A multifrequency LCR meter manufactured by Yokogawa Hewlett-Packard Co., Ltd. FIG. 5 shows a vector ammeter employed as an impedance measuring instrument which uses such a wideband half bridge circuit.
Referring to FIG. 5, an AC signal source 1 produces an AC signal which is passed through a source resistor 2 and divided into three components. One of these components is fed to a vector voltage ratio meter 17 through a first path 4. Another component is furnished to the inverting input terminal of an amplifier 7 through a device 3 being measured. The remaining component is sent to a signal distributing circuit 9 through a path 5.
The amplifier 7 and a feedback resistor 8 form a current-to-voltage converter circuit to which an electric current is applied via the measured device 3 and a reference resistor 6. The converter circuit converts the input current into a voltage that is fed to one terminal of each one of phase-sensitive detectors 10 and I1. The other input terminals of the detectors 10 and 11 are excited with driving signals which are delivered from the signal distributor 9. These driving signals have the same frequency as the aforementioned AC signal and are 90 out of phase with each other.
The outputs from the phase-sensitive detectors 10 and 11 are smoothed and amplified by integrators 12 and 13, respectively, and applied to one terminal of modulators 14 and 15, respectively. The modulators 14 and 15 modulate the 90.degree. out-of-phase signals which are applied to the phase-sensitive detectors 10 and 11. The modulated signals are applied to an adder 16. The output from the adder 16 is fed to the inverting input terminal of the amplifier 7 via the reference resistor 6.
The network is designed so that all the electric current flowing through the measured device 3 passes through the reference resistor 6. The inverting input terminal of the amplifier 7 forms a virtual ground point.
The vector voltage ratio meter 17 receives the output from the adder 16 and the AC voltage which is passed through the path 4 to energize the measured device 3. The meter 17 calculates the vector ratio of the output from the adder 16 to the AC voltage passed through the path 4. The calculation of the vector voltage ratio is carried out by the above-described HP 4274A meter or other instrument and is well-known. Therefore, description of the calculation is omitted herein. Of course, a vector ratio is expressed in terms of the ratio of one of two inputs to the other and their phase difference.
Let V.sub.3 be the AC voltage for energizing the measured device 3 and V.sub.16 be the AC voltage delivered from the adder 16. The impedance Z of the measured device 3 is given by ##EQU1## provided the resistance R.sub.6 of the reference resistor 6 is a parameter. In the above equation, the inside of [ ]expresses the vector voltage ratio. Similarly, by taking the vector ratio of current I.sub.3 flowing through the measured device 3 to V.sub.3, we have ##EQU2## Generally, the vector voltage ratios of V.sub.3 and V.sub.16 to the third input V.sub.9 supplied from the signal distributor 9 are calculated. Then, the vector voltage ratio of V.sub.16 to V.sub.3 is calculated. In case of Eq. (2), the vector current is admittance.
The prior art techniques described thus far have the following disadvantages. Since a vector voltage ratio meter is needed, the cost and the space requirements are great. Furthermore, the vector voltage ratio cannot be calculated until the bridge portion shown in FIG. 5 excluding the vector voltage ratio meter stabilizes. Therefore, a long time is taken for the measurement. Also, it is difficult to stabilize the bridge portion at radio frequencies.
Accordingly, it is an object of the invention to provide a small-sized, high-speed, inexpensive vector ammeter which uses a digital signal processing unit and thus dispenses with a vector voltage ratio meter, thereby eliminating the foregoing disadvantages.